2262
G. D.
[CONTRIBUTION FROM
THE
FASMAN AND
E.
R.BLOUT
Vol. 82
LABORATORY OF THE CHILDREN'S CANCERRESEARCH FOUNDATION AND THE I~ARVARD MEDICAL SCHOOL]
The Synthesis and the Conformation of
Poly-L-serine
and Poly-0-ace tyl-L-serine'.'
BY G. D. FASMAN AND E. R. BLOUT RECEIVED SEPTEMBER 14, 1959 Poly-L-serine and poly-0-acetyl-L-serine have been synthesized with degrees of polymerization ( D P ) slightly above 100. The deacetylation of poly-0-acetyl-L-serine to poly-L-serine using sodium methoside does not result in chain cleavage. Conformational studies on poly-0-acetyl-L-serine by means of infrared spectroscopy and optical rotatory dispersion indicate that this polymer exists in either a fi (extended) or random conformation depending on the solvent. KO evidence for an CYhelical form has been found. A reversible random +. p-conformational change has been observcd with poly-0-acetyl-Lserine accompanied by a n optical rotation change of approximately 100" at Lads. Poly-L-serine exists in random conformation in aqueous solution and in the solid state. No evidcnce has been found for an a-helical conformation with this DP of poly-L-serine.
I. Introduction The synthesis of poly-DL-serine was reported by Synthetic high molecular weight polypeptides Frankel, et al.,g in 1952 from the polymerization of However, (derived from a-amino acids) have proved to be 0-acetyl-m-serine-N-carboxyanhydride. useful models for the investigation of protein con- the N-carboxyanhydride (NCA) used was not formation, but heretofore the only water-soluble crystalline and only low molecular weight polypolymers that have been available for such studies mers were obtained. 0-Acetyl-DL-serine-NCA was have had one of the following disadvantages: obtained crystalline by MacDonald and Tullock10 low molecular weight, ionic side groups or have and polymerized, but the complete deacetylation been derived from imino acid^.^^,^,^ In this paper of the resulting polymer was not achieved. Earlier we describe the synthesis of a non-ionic, water- the synthesis of poly-0-carbobenzoxy-DL-serine soluble polypeptide, poly-L-serine (and poly-0- with degree of polymerization (DP) of 5 was reacetyl-L-serine) , of molecular weight sufficiently ported." The blocking group was removed, high to stabilize the a-helical conformation in but the polymer was not characterized. Polywater-insoluble polypeptides. The work recorded 0-methyl-DL-serine also has been synthesized. lZa in this paper also describes investigations of the 11. Synthesis conformation of these polymers by means of inPoly-L-serine and poly-DL-serine have been synfrared spectroscopy and optical rotatorydisper~ion.~~ thesized by two different methods. First, polyPoly-L-serine is the simplest water-soluble, hymerization of 0-acetyl-serine-NCA of the respecdroxyl-containing poly-a-amino acid. The imtive L- or DL-amino acids, followed by deacetylaportance that serine plays in protein chemistry has been recognized since Lipmann4 in 1932 iso- tion gave the desired polymer. Second, direct of the L- or DL-serine-NCA yielded lated phosphoserine from casein and vite,llin hy- polymerization poly-L-serine and pdy-DL-SerinC, respectively. The drolysates. More recently Sanger and Williams5 preparation of the crystalline free hydroxy NCA's have isolated serine phosphopeptides (SerP) from of the monomers for this second synthesis is dethese sources. Peptides containing six units, (SerP)a, from phosvitin and three units from casein scribed in this paper. A note describing the prepaof ~ ~ - s e r i n e - N Chas A appeared recently. lZb have been separated from their respective hydroly- ration polymerization of these anhydrides and the sates. Perlmann6 has investigated the types of The structure of the polymers is the subject of a subsephospho-ester bonds in proteins, and has sug- quent communication.13 gested that serine plays an important role in such L-Serine and DL-serine were 0-acetylated by the structures. I n addition serine has been implicated Sheehan method14 followed by reaction with in the active sites of several enzymes.' A protein phosgene in either dioxane or ethyl acetate. In containing 40.77' serine has been isolated from the both cases the crystalline anhydrides were obtained green lacewing fly Chrysopa.8" from benzene, ether or ethyl acetate-hexane solu(1) T h i s i s Polypeptides XXVII. For t h e preceding paper in this tions. Polynierization of 0-acetyl-DL-serine and series see E. R. Biout and L. Stryer, PYOC. Y a f l . A c e d . S c i . , 46, 1591 0-acetyl-L-serine-NCA was carried out in the fol(1959). Alternate address of E. R. Blbut: Chemical Research Laboralowing solvents : dioxane, nitrobenzene, dimethylt o r y , Polaroid Corp., Cambridge 39, hlass. formamide, pyridine and methylene dichloride. (2) T h i s work bas been supported by t h e Otlire of the Surgeon General, Department of the Army. The polymerizations mere initiated with either (3) (a) A. Berger, J. Kurtz and E. Katchalski, Tins J O U R N A L , 7 6 , sodium methoxide, triethylamine or diethylamine 35.52 (1954); (b) J. K u r t z , G. D. Fasman, A. Bergerand E Katchalski. ibrd., 8 0 , 393 (1958); (c) E. R. Bluut and G. D. Fasman, " R e c e n t Advances in Gelatin and Glue Research," Pergamon Press, I.ondon, 1958, p. 122; (d) G. D. Fasman a n d E. R . Blout, Abstracts IV International Biochemistry Congress, Vienna, 1958, p. 1. Presented i n part a t the 136th Meeting of the American Chemical Society, Atlautic City, N. J., Sept. 13-18, 1959. (4) F. Lipmann and P. A. Levene, J . B i d . Chem., 98, 107 (1932); P. Lipmann, Nafurwissenschaffen,21, 236 (1033); F. Lipmann, Bioc h a m %.. ( 5 ) 1'. (1959). (6) G. (7) H.
262. 3, 9 (1933). Sanger and J. Williams, Biochem. B i o p h y s . A c f o , 33, 294 I'erliuann, Advances iir Proteiw Cheiisislry, 10, 1 (1955). Xeurath. i b i d . , 12, 319 (1957).
(8) (a) F. Lucas, J. T. B. S h a w a n d S. G . Smith, .Valirve, 179, 906 (1957); ( b ) K. D. Parker and K . hf. Rudall, i b i d . , 179, 905 (1957). (9) 31.Fraokel, 3.T. Bteuer and S. Cordova, Exgerien!!a, 8, 1 (1952); 31. Frankel, S. Cordova a n d hf. Breuer, J. C h e m . S o r . , 1991 (19.51). (10) R. N. RiacDonald and C. W.Tillluck, U. S . Patent 3,030,423 (1953). (11) M. Frankel and M.Holmao, J C1iem. Soc., 2736 (19.52). (12) (a) W. E. Hanby and Yates in "Synthetic Polypeptides," by C. I€ Bamford, A. Elliott end W. E. Hanhy, Academic Presz, Iiic., N e w York, N. Y., 1'331:; (b) A. Berger. J. K u r t z , T, Sadch, A . Y a r o n , R . Arnon and Y. Lapidoth, Birll. Res. Counc. o f I s r a e l , 7 8 , 98 (1058).
(13) To be published. (14) J. C.Sheehan and G. 1'. HICSS, THISJ O U R N A L , 77, 1067 (1955).
May 5, 1960
POLY-L-SERINE AND POLY-0-ACETYL-L-SERINE
2263
opposite optical configuration has been shown to greatly decrease the molecular weight of the resulting polymer;18 this explanation may be a valid one. One additional hypothesis has been considered to explain the relatively low molecular weights obtained in the polymerization of serine-NCA. This is based on the finding (see below) that poly0-acetyl-L-serine exists in the @-conformation. It has been shown that the rate of polymerization of polypeptides in the 0-conformation is very much slower than that of such polypeptides when in the helical conformation.18 If, as shown below, TABLE I poly-0-acetyl-L-serine exists only in a /3 or in a PHYSICAL PROPERTIES OF POLY-0-ACETYL-L-SERINE AND random conformation, this may be the explanaPOLY-L-SERIXE tion of the lower molecular weights obtained with Poly-0-acetyl-Lthis amino acid compared with others which exist as serine' Pol y-L-serineb [vi ( ~ 1 2 6 ~ 4 ~ [?I [UI2%46i Solvent helices in the polymerizing medium. 0.19 $31' - 9' DCA That no degradation took place during the de0.10 -2gh Hz0 acetylation with sodium methoxide is evident from .08 -25' 8 Murea the titration data (Table I) where the degree of .06 - 7' 10 M L i B r polymerization was found to be approximately - 14i HzO-dioxane (1:2) the same by perchloric acid titration of the polymer MW, DP, MWn DPn before and after the deacetylation. The degree 15,8OOc 122 10,6OOc 122 of polymerization of the poly-L-serine by both amino 8,300d 95 end groups and carboxyl end groups also gave relahIW, DP, MWV, DPw tively good agreement. Since there are approxi16,800E 130 11,300" 130 mately the same amount of carboxyl and amino Sample #G-l7; other samples used i n this study GF-E- groups, these data may be cited as evidence that 39-2, tlaPlo 0.14, 0.2% dichloroacetic acid (DCA): GF-5-33, very little 0 + N shift of the acetyl group takes 0.14 (0.2y0 DCA). Sample G-17-2. e End group during the polymerization, and this is prob~ titration with perchloric acid; M. Sela and A. B e r g e r , T ~ ~place 77, 1893 (1955). Carboxyl group titration with ably not the cause of the relatively low molecular JOURNAL, sodium methoxide using thymol blue as indicator. e Es- weight of the polymers.
a t anhydride :initiator ratios (A/I) which varied between 1 and 200. The polymer is insoluble in most organic solvents, is slightly soluble in dimethylformamide and soluble in dichloroacetic acid, trifluoroacetic acid, acetic acid or mixtures of these acids with chloroform or ethylene dichloride. The poly-0-acetyl-serines were deacetylated in nitrobenzene or dimethylformamide solutions using sodium methoxide to yield water-soluble polyserines. Some physical constants of poly-0acetyl-L-serine as prepared by this method and of poly-L-serine are shown in Table I.
timated from viscosity data from P. Doty, J. H. Bradbury 78,917 (19.56). f Concn. and A.M. Holtzer, THISJOURNAL, lye-. 0 Concn. 0.4%. Concn. 0.3%. Concn. 0.6%. 1 When these values were corrected for the refractive index of the solvents used, the corrections were so small that this correction has been omitted.
The molecular weights obtained were in the range 10,000to 15,000,that is, a degree of polymerization (DP) of approximately 125. Although many polymerizations were run using the techniques previously found suitable for obtaining molecular weights of the order of 100,000 to 1,000,000 with other a-amino acid N-carboxyanhydridesi5 (DP > 1000) no higher molecular weight polymers than those indicated were obtained. One postulate that can be offered to explain this situation is that an O+N shift16 of the acetyl group takes place under the basic conditions used and that this shift with the concomitant formation of an amide group acted as an effective terminator of the polymerization reaction. The fact that the molecular weight of these polymers could not be increased by the carbodiimide method" favors this explanation. When triethylamine was used as initiator, which would not be expected to cause the 0 + N shift, no higher molecular weights were obtained. The possibility that the L-serine used in these experiments was contaminated by traces of D-Serine cannot be eliminated, since traces of an optical isomer of the (15) E. R. Rlout aud R. €1. Karlson, THISJOURNAL, 78, 941 (1956). (16) Y. Shalitin, Ph.D. Thesis, Hebrew University, Jerusalem, Israel, 1958. (17) E. R . Blout and 11. E. DesRoches, THISJOURNAI., SI, 370 (1959).
111. Experimentalla Infrared Measurements.-All infrared measurements were performed on a Perkin-Elmer model 21 double beam spectrometer using a sodium chloride prism. The samples for infrared spectral measurements were prepared either as (a) films cast on silver chloride plates; oriented films were prepared by unidirectional stroking of a viscous solution on silver chloride plates until i t was dry; (b) as dispersion in KBr disks under 120,000 p.s.i. pressure; or (c) solutions with polymer concentrations about 2%. Optical Rotation Measurements.-Optical rotatory dispersion measurements were made with a Rudolf high precision photoelectric polarimeter model 80S, using a General Electric H100-A4mercury lamp as light source. All measurements were made a t 25.0 0.1'. Conceiitrations were usually of the order 0.4%. 0-Acetyl-L-serine-N-carboxyanhydride .-0-Acetyl-Lserine.HCl'Q (1 g.) was suspended in dry dioxane20(30 cc.), stirred and phosgene bubbled in for 0.5 hour, maintaining the temperature a t 65". Nitrogen then was bubbled through the clear solution until free of phosgene. The solvent was removed a t reduced pressure, the temperature maintained a t 40°, leaving a clear oil. The oil was dissolved in xnhydrous ethyl acetate ( 2 5 cc.), dry hexane (40 cc.) added until opalescence, the solution filtered, more hexane (165 cc.) added and then the solution was left a t -30". On standing overnight, crystallization had begun and more hexane (150 cc.) was added. The crystals were filtered, yielding 0.90 g., 96% theoretical. Recrystallization from anhydrous ethyl acetate (25 cc.) and n-hexane (290 cc.) gave 0.73 g. of crysAnal. Calcd. for CsH7N05: tals, 78'30, m.p. 50.0-51.0'. C, 41.6; H, 4.08; N, 8.09. Found: C, 41.7; H, 4.1; N, 8.0. (18) RT. Idelson and E. R. Blnut, ; k i d . , 80, 2387 (1958). (Ma) All melting points corrected. (19) J. C. Sheehan, M. Goodman and G. P . Hess, i b i d . , 78, 1307 (1956). (20) L. F. Fieser, "Experiments in Organic Chemistry," 2 n d e d . , D. C. Heath and Co.,Boston, Mass., 1941, p. 361.
2264
G. D. FASMAN A N D E. R. BLOUT
O-Acetyl-DL-serine.Hcl.-DL-Serine (7 g.) was dissolved in glacial acetic acid (300 cc.) and the solution saturated with HCl a t 0'. After standing 15 hours a t room temperature, the solvent was removed a t reduced pressure. This procedure was repeated and the resulting crystals were recrystallized from ethyl alcohol (60 cc.) and ether (60 cc.); yield l o g . , 8970, m.p. 158-162' dec. Anal. Calcd. for C6H&04~HC1:C, 32.6; H, 5.48; X, 7.62. Found: C,32.7; H , 5 . 7 ; hT,i . 6 . 0-Acetyl-Dr -serine-N-carboxyanhydride .-0-Acetyl-DLserine.HC1 was treated in the same manner as 0-acetyl-Lserine.HC1. The recrystallized product gave an SOY0 yield, m.p. 62.5-63.5'. Anal. Calcd. for C ~ H T N O ~C,: 41.6; H,4.08; N,8.09. Found: C,41.5; H,4.0; N,8.1. L-Serine-N-carboxyanhydride.-L-Serine (4 g.) was suspended, by stirring, in anhydrous ethyl acetate (400 cc.), and phosgene bubbled in for 2.5 hours while the solvent was refluxed. Nitrogen was passed through the solution until it was phosgene free. The solvent was evaporated under reduced pressure, keeping the temperature below 40'. The resulting oil was dissolved in anhydrous ethyl acetate (125 cc.), n-hexane was added t o opalescence (10 cc.) and the solution filtered and placed at -30": an amphorous precipitate resulted. The solvent was removed a t reduced pressure, the temperature kept below 40". The residue was extracted four times with anhydrous benzene a t 60' (100 cc. each). The benzene extract yielded crystals at 10". The extracted oil crystallized on standing a t -30'; combined yield 4.85 g., 970j0, m.p. 115' dec. Anal. Calcd. for CdHSN04: C, 36.6; H, 3.8; N, 10.7. Found: C, 36.4; H , 3.8; N, 10.2. DL-Serine-N-carb0xyanhydride.-DL-Serine (10 g.) was suspended in dry dioxane20 by stirring and phosgene was bubbled in for 3 hours, maintaining the temperature at 65'. Nitrogen was passed through the solution until it was phosgene free. The dioxane was removed unter reduced presThe resulting sure with the temperature kept below 40 oil was dissolved in anhydrous ethyl acetate (100 cc.), and n-hexane (250 cc.) was added and crystallization began at -30". The crystals were filtered yielding 4 . 3 g., 3495, m.p. 82-83'. The material was recrystallized from anhydrous ethyl acetate and hexane for analysis. It crystallized with half a mole of ethyl acetate. Anal. Calcd. for C4HaN04, 1/2C4H802:C, 40.9; H , 5.15; N, 7.96. Found: C, 41.1; H, 5 . 2 ; N, 7.9. The anhydride is soluble in dioxane, dimethylformamide, ethyl acetate and water, and insoluble in nitrobenzene. Poly-0-acetyl-t-serine .-0-Acet yl-L-serine-N-carboxyanhydride ( 2 9.) was dissolved in dry nitrobenzene (50 cc.) and initiated with triethylamine (1.12 cc. of 0.4 kfin benzene). The polymerization was allowed to proceed for two days. The solution was then poured into anhydrous ether (300 cc.) with stirring, causing the polymer t o precipitate. The mixture was centrifuged and washed three times with ether. The polymer was suspended in dioxane and lyophilized, yielding a white powdery solid; weight, 1.1 g., 7570 yield. The viscosity was measured in 0,2y0 dichloroacetic acid using material diied a t 100' in vacuo, vsp/o 0.19. The polymer was insoluble in most organic solvents. I t was soluble in dichloroacetic acid, trifluoroacetic acid, and mixtures of these acids and other organic solvents such as chloroform, and partially soluble in nitrobenzene. Poly-L-serine.-Poly-0-acetyl-L-serine (1 g .) was partially dissolved in nitrobenzene (100 cc.) by stirring and 1.2 moles of sodium methoxide (22 cc. of 0.426 N NaOCHa) added causing immediate precipitation. The solution was stirred overnight and then poured into anhydrous ether (500 cc.) to isolate the polymer. The solution was centrifuged and the polymer washed three times with anhydrous ether. The solid was dissolved in water ( 5 cc.), giving a yellowish solution, dialyzed versus water until i t became colorless, and lyophilized yielding a white fluffy powder, weight 0.35 g . , 5 5 5 , specific viscosity, vsp/o0.10 ( c 0.2% in HzO). The material requires drying t o remove the water completely. The polymer is soluble in: H10, dichloroacetic acid, trifluor, iacetic acid, hydrazine and ethylene chlorohydrin. Triethylamine does not deacetylate 0-acetyl-r,-serine or poly-0acetyl-L-serine, If the dialysis step is omitted a hygroscopic material is obtained. Poly-0-acetyl-DL-serine .-O-Acctyl-DL-seriiic-lY-carboxyanhydride (5.0 g , ) was dissolved in nitrobenzene (125
.
Vol. 82
cc.) and initiated with sodium methoxide (1.36 cc. of 0.426 N NaOCHa). The solution gelled within a few minutes and was allowed to stand overnight. It was then poured into anhydrous ether with stirring causing a heavy precipitate to deposit. The solution was centrifuged, and the precipitate washed three times with ether. The polymer was suspended in dioxane and lyophilized, yielding a white fluffy solid, weight 3.4 g., 91Y0, vlapio0.11 (c 0.2% in dichloroacetic acid). The polymer was soluble in dimethylformamide as well as the solvents which dissolved poly-0-acetyl-L-serine. Poly-DL-Serine.-Poly-0-acetyl-DL-serine was dissolved in dimethylformamide and 1.2 mole equivalent of sodium methoxide (0.426 N)was added with stirring, causing precipitation of the polymer. The polymer was worked up as above for poly-L-serine.
IV. Conformational Studies In order to determine the conformations of poly0-acetyl-L-serine and poly-L-serine in the solid state and in solution, the methods of infrared spectroscopy and optical rotatory dispersion have been used. In addition X-ray diffraction has provided confirmation of some of the conclusions from the infrared and optical rotation data. A. Poly-0-acetyl-L-serine.-Infrared Results.The infrared spectra of poly-0-acetyl-L-serine were obtained in the solid state from KBr disks and oriented films. Solutions were studied in the solvents listed in Table I1 which summarizes some of the infrared data obtained on this polymer. The infrared spectra of an oriented film of poly0-acetyl-L-serine are shown in Fig. 1; this polypeptide shows amide I (amide carbonyl) absorption a t 1637 cm. with perpendicular dichroism, and amide I1 absorption a t 1517 with parallel T A B L E 11 INFRARED DATAo s POLY-0-ACETYL-L-SERIXE~
F r e q u m c y , cm - 1 .4mide I Amide I 1
Sample Ytate
Solid, KBr disk"
1633 1650(~) 16371 1705(S)'i 1827
1512 1545(s) Solid, oriented (from TFA)h 1517~' 15-&5(~) Soln., 27% DC.I-73Sb EI)Cc 1514 1545(s) Soln., 25% DC.4-75r/, CHCI,' 1630 1520 1655(s) 1550(s) Soln., 40% TFX-607& EDC' 1642-1615 1531 1517(s) Soln., 557, TFA-45% EDCc 1650-1640 1530 (split) l545(s) Solil., 55% DCA-45% EDC' 1615 1525 Sample no. 5-25. * Sample no. G-31. Sample no. G17. Symbols used: s = shoulder, TFA = trifluoroacetic acid, DCA = dichloroacetic acid, E D C = ethylenedichloride, I = perpendicular dichroism, !I = parallel dichroism.
dichroism. The above irifrared data indicate that poly-0-acetyl-L-serine exists in the p (extended) conformation in the solid state. In addition to the strong evidence provided by the dichroism of the oriented films, further evidence is provided for the existence of an anti-parallel R-structure by the presence of the band of 1705 cin.-1.31 In addition, preliminary X-ray diffraction results indicate that fibers of poly-0-acetyl-L-serine exist as a crossed /3 conformation.22 This structure was also reported for a protein containing 40.7';rc serine.sb The fact ( 2 1 ) T. Xliyazawa .I. Che?n. P h y s , in press
(22) C. Johnson aud C. Cohen, private communication
May 5, 1960
2265
POLY-L-SERINE AND POLY-0-ACETYL-L-SERINE
\
360
9
280
:b I60
15
I
I800
1700
,
l lMl0
I
I IMO
,
I 1400
,
l
Rw
,
l
1
1200
cm.'
Fig. 1.-The infrared spectra of an oriented film of poly-0acetyl-L-serine. The film was oriented by unidirectional rubbing of a concentrated solution of the polypeptide in trifluoroacetic acid: -, electric vibration direction parallel to orientation direction; ------- , electric vibration direction perpendicular to orientation direction. Note presence of perpendicular amide I absorption at 1637 em.-' and parallel amide I1 absorption a t 1517 cm.-l.
t
8ol
40
-
4
0
t
3600
l
,
,
4400
,
, 5200
,
, I
I
6000
that poly-0-acetyl-L-serine is not soluble in weakly IN A. hydrogen bond-forming organic solvents such as Fig. 2.-The rotatory dispersions of poly-0-acetyl-L-serine chloroform, but requires the addition of strongly hydrogen bond-forming organic solvents such as in various solvent compositions : A, dichloroacetic acid dichloroacetic acid or trifluoroacetic acid to promote (DCA): ethylenedichloride(EDC) (1:3); B, DCA: CHCla solubility, by breaking interchain hydrogen bond- (1:3); C, DCA: E D C (1:l); D, DCA: CHCla (3:l). The ing, suggests that this polymer exists as a highly data for 100% DCA were practically identical with curve D. hydrogen-bonded structure in the solid state. This The optical rotatory data for poly-0-acetyl+ is consistent with the assignment of a &structure. serine in DCA-chloroform solution and in DCAIn addition, the fact that the principal amide I ethylene dichloride solution a t various solvent comfrequency of the solid polymer (in KBr disks) lies positions are shown in Fig. 2, where [ a ] 5 ~ is, plotted a t 1633 cm.-' provides further evidence for the as a function of wave length. Over the wave assignment of a 0-conformation and evidence length range studied simple optical dispersion is against the existence of a helical conformation, in observed in these solvents. which the amide I frequency usually lies in the TABLE 111 region 1650 to 1655 cm.-1.23 Less extensive infrared measurements have been ROTATORY DISPERSIONRESULTSFROM POLY-0-ACETYL-LSERINE (G-17) carried out on poly-0-acetyl-m-serine, but the data obtained are similar to those obtained with poly-0- -Solvent$' 6 DCA % other qolvent [a]%46 XC b o ( X 0 = 212) acetyl-L-serine. 100 4- 31 120 -80 Optical Rotatory Dispersion Data.-As indi50 50 CHCla 36 120 -80 cated above, the insolubility of this polymer in 50 50 E D C 44 144 Non-linearplot most organic solvents limits the possible optical 25 75 CHC13 $119 214 0 rotatory dispersion studies. In DCA solution poly25 75 E D C +120 214 0 0-acetyl-L-serine shows simple rotatory dispersion, c 15 85 CHClr 4-127 that is, the data fit the one-term Drude equation. 10 90EDC 4-159 The results in this solvent and in mixtures of this A, is derived by the modified Drude plot" and bo from the solvent with chloroform and ethylene dichloride are Moffitt equation." J. T. Yang and P. Doty, THISJOURNAL, shown in Table 111. 79, 761 (1957). * W. Moffitt, J . C h m . Phys., 25, 467 In all solutions listed above, the data fit a simple (1956). Solution gelled and no dispersion measurements one-term Drude equation, and when plotted in the were made. manner suggested by MoffittZ4using XO = 212, the However, from the values of [a1646 listed in Table coefficient of the second term is approximately 111, it is seen that there is a large change in optical zero, indicating the absence of anomalous disper- rotation as a function of solvent composition. sion of the type associated with the a-helical con- DCA solutions of poly-0-acetyl-L-serine have been formation. We therefore conclude that in the sol- titrated with chloroform and the data are plotted vents studied no helical conformation is present. It in Fig. 3. The change in [ a 1 5 4 6 is approximately will be noted that the value of XO shows considerable 100°-more than twice that observed previously variation as the solvent composition is changed. with polybenzyl-L-glutamatewhen the same solvent compositions were examined.2s However, as noted (23) H. Lenormant, A. Baudras and E. R. Blout, THIS JOURNAL, 80,
+ +
E
6191 (1958).
(24) W. Moffitt, J . Chcm.P h y s , 86, 467 (1956)
(25) E. R . Blout, P. Doty and J. T. Yang, THIS JOURNAL, 79, 749 1957).
G. D. FASMAN AND E. I
